A light beam emitted practically by any powerful laser-diode source has in its cross section two mutually orthogonal directions where angular divergences of the beams in both directions are different. Conventionally, the direction with the maximal divergence is known as a fast axis, and the direction with the minimal divergence is known as a slow axis. The above phenomenon creates problems in some optical devices where homogeneity of a beam in mutually perpendicular directions of its cross section is a critical factor. One such optical device is a laser-diode bar which nowadays finds a very wide application in the fields of material processing, medical instrumentation, etc.
A diode bar is a high-power semiconductor laser (laser diode), containing a one-dimensional array that consists of broad-area emitters. They typically may contain 20 and 50 emitters. Each such emitter may have a width, e.g., of about 100 μm and pitch P of 500 μm.
A commercial laser-diode bar available on the market may have a set of laser resonators with the length of the order of 1 cm and may generate a resulting power of the order of tens of watts or even up to ≈200 W. Some prototypes even reach several hundreds of watts (possibly with a reduced lifetime).
Most diode bars operate in the wavelength region from 780 to 980 nm and are used for pumping neodymium, erbium YAG lasers, and for pumping erbium-doped or ytterbium-doped high-power fiber lasers and amplifiers.
However, a specific pattern of radiation from the laser-diode bars makes it difficult to control the shape of the emitted beam. The problem occurs because of the aforementioned different divergence of the beam in the directions of the slow and fast axes. This divergence leads to interference between individual beams at a short distance from the surface of the emitters, and since the emitters are located at a close distance from each other, there is not enough room for placing beam correction means.
Heretofore many attempts have been made to solve the above problem in laser diode arrays. For example, U.S. Pat. No. 5,268,922 issued in 1993 to J.-C. Fouere and C. Metreaud discloses a simple optical collimating device for a single laser diode in the form of a single aspheric lens assembled integrally with a laser diode. A disadvantage of such a device is that in order to compensate for divergence difference on orthogonal axes of the beam cross section, the aforementioned aspheric lens should have a complicated custom design. Furthermore, the device of U.S. Pat. No. 5,268,922 is not applicable to laser diode arrays because of even higher spatial angular anisotropy and partial overlapping of beams emitted by adjacent diodes in the same plane.
U.S. Pat. No. 5,636,059 issued in 1997 to J. Snyder discloses an assembly of two aspheric, e.g., cylindrical, lenses with mutually perpendicular generatrices of refractive surfaces. Each lens functions for a separate axis, i.e., one lens reshapes the beam on the fast axis, while the other reshapes the beam on the slow axis. A similar system may consist of two reflective surfaces, e.g., mirrors, similarly located with respect to each other. Although such a system differentiates beam reshaping functions along different axes, it has a macroscopic, i.e., extended size and therefore presents a problem for a matrix-type arrangement of light sources, particularly for those with small steps.
U.S. Pat. No. 5,056,881 issued in 1991 to Terry Bowen, et al. describes an assembly of a laser diode with at least one optical holographic element located at the output of the laser diode. This system circularizes the beam, collimates it, and removes chromatic aberration. In order to ensure sufficient power compatibility, the holographic element of such a system should be manufactured from a very durable and energetically efficient material such as quartz, which makes the system as a whole relatively expensive. If, on the other hand, the system employs inexpensive, e.g., plastic, replicas for the holographic element, it would have practical applications limited only to low-power sources. Furthermore, similar to the system of U.S. Pat. No. 5,636,059, the system with holographic elements is inapplicable to matrix-type sources.
U.S. Pat. No. 4,609,258 issued in 1986 to Iwao Adachi, et al. discloses a collimating system for laser diodes which utilizes a prismatic-type collimator. A disadvantage of this system is that it generates chromatic aberrations inherent in any prismatic systems. Despite the fact that the system itself consists of many components, compensation of the aforementioned aberrations requires the use of additional optical components. As a result, the system has increased overall dimensions.
U.S. Pat. No. 5,541,774 issued in 1996 to R. Blankenbecler describes so-called gradient optical elements. These optical elements can replace various cylindrical, conical, and other aspheric elements used for collimating and beam reshaping. Such optical systems are compact, compatible with matrix-type light sources, but are complicated in structure and expensive to manufacture. However, the range of commercially available materials is limited, and therefore gradient optical elements can be manufactured with limitations dictated by wavelengths and output power of the light sources compatible with such optical systems. Another disadvantage of gradient optical elements in light of their application to beam shaping is that they have a limited range of the refractive index variation, which sometimes is insufficient for precise reshaping of the light beam.
U.S. Pat. No. 5,825,551 issued in 1998 to William A. Clarkson discloses a beam shaper utilizing a principle of multiple re-reflection in the system of two parallel reflective surfaces (including the case of total internal reflection). A main disadvantage of such a system is interference of reflected beams which causes spatial modulation of radiation resulting in its inhomogeneity.
Another similar system is described in U.S. Pat. No. 5,808,323 issued in 1998 to Werner Spaeth, et al. This system consists of a cylindrical lens common for a line of photo diodes and two mirrors. The use of a cylindrical lens introduces into the system all disadvantages described above with regard to the systems utilizing aspheric elements. Furthermore, the use of a single cylindrical lens for the entire strip of the diodes does not prevent the adjacent beams from interference and does not allow individual adjustment of beams emitted by individual light sources.
The above disadvantages are partially solved in a fault tolerant optical system described in U.S. Pat. No. 5,369,659 issued in 1994 to Horace Furumoto, et al. The system consists of the following elements arranged in sequence: a laser diode array, two lenslet arrays (collimating and correcting), and an assembly of directing and focusing optics. However, this system comprises a macroscopic workbench which collimates and corrects individual beams as a whole without addressing the aforementioned fast and slow axes individually, i.e., without separate adjustment of beam divergence in the aforementioned directions. Thus, such a system does not compensate for faults resulting from non-uniform divergence of the beam in the directions of slow and fast axes. This system rather differentiates two functions of the beam shaper, i.e., one lens array is used for correcting the optical faults where the second lens array performs fill-factor enhancement. Another disadvantage of the system of U.S. Pat. No. 5,369,659 is that it consists of a plurality of individual lenses produced, e.g., by laser milling. In other words, each array has a composite structure and consists of a plurality of individually manufactured or processed lenses. Moreover, as is stated in the aforementioned U.S. patent, in the manufacturing process with laser milling each individual lens is associated with an individual laser. Thus, the manufacturing process is complicated, expensive, time-consuming, and may involve custom design. In other words, the device of U.S. Pat. No. 5,369,659 cannot be produced in a single manufacturing step such as molding or etching.
U.S. Patent Application Publication No. 20050105189 (inventor: A. Mikhailov) discloses an arrangement for optical beam transformation having at least one light source which can emit at least one light beam with the at least one light beam having a greater divergence in a first direction (Y) than in a second direction (X) at right angles to it. The system further comprises a collimation means, which can at least reduce the divergence of the at least one light beam in the first direction (Y), and an apparatus for optical beam transformation, which is arranged downstream from the collimation means in the propagation direction (Z) of the at least one light beam, with the apparatus being such that the divergence of the at least one light beam passing through the apparatus in the first direction (Y) is interchanged with the divergence in the second direction (X) at right angles to it, and such that the cross-sectional area of the at least one light beam is reduced in the apparatus for optical beam transformation.
U.S. Pat. No. 6,407,870 issued in 2002 to I. Gurevich, et al. discloses an optical system comprising a first array of individual beam shaping elements and a second array of beam shaping elements placed between a light source, e.g., a linear array of individual laser diodes, and a reshaped beam receiver, e.g., an optical fiber cable. The inhomogeneous beams emitted from the laser diodes are passed in sequence through the first and second stages so that the first stage reshapes the cross section of the beam, e.g., in the fast-axis direction, and the second stage reshapes the cross section of the beam, e.g., in the slow-axis direction. As a result, the output beams of the system may have a cross section reshaped to any desired configuration, e.g., suitable for inputting into the optical-fiber cable and having divergences individually adjusted in mutually perpendicular directions.